1. Field of the Invention
The invention generally relates to a method and apparatus for transferring a photographic image by diffusion from a donor to a receiver. More particularly, the invention relates to a method and apparatus in which the image is transferred from a donor, treated with liquid activator, to one side of a dry receiver without wetting an opposite side of the receiver.
2. Description Of The Prior Art
In one form, such as disclosed in U.S. Pat. No. 4,223,991, image-transfer processing involves the use of separate donor and receiver sheets. The emulsion-bearing donor is exposed imagewise and is soaked in liquid activator to develop the latent image. The activated wet donor is placed in registration with a dry receiver and the two sheets are fed through a pair of pressure rollers, where they are pressed together, beginning the diffusion transfer of the developed image from the donor to the receiver and wringing excess activator from the donor. When image transfer is completed, usually after several minutes, the donor is peeled from the image side of the receiver to reveal a finished print.
Some of the excess activator removed from the donor collects as a meniscus at the entry nip of the rollers. If the meniscus is allowed to remain in the entry nip after the donor and receiver are fed through the rollers, it will evaporate, leaving a deleterious residue of highly corrosive activator salts at the entry nip, which may eventually damage the rollers or cause them to adhere to one another. When successive pairs of donors and receivers are fed through the rollers, a new meniscus will be transferred to the rollers each time a pair of donor and receivers is introduced into the rollers, causing the roller that contacts the back side of a receiver, i.e. the side of the receiver opposite its image side, to spread activator as a non-uniform thin covering onto the back side. This is undesirable for a number of reasons. For example, the formation of dried activator salts on the back side of a receiver is undesirable from a "cosmetic" standpoint and, in view of the high alkalinity of the salts, touching of them should be avoided. Moreover, activator salts on the back side of a receiver, prevents stacking of several receivers because the salts may transfer from the back side of one receiver to the image side of an adjacent receiver, possibly damaging the image side. More importantly, however, the non-uniform wetting of the back side causes a temperature difference between the wet and dry areas of the back side due to evaporative cooling of the wet areas. This temperature difference produces undesirable differences in image density between the wet and dry areas.
Therefore, there exists a need to keep the back side of each receiver dry during image-transfer processing of successive pairs of donors and receivers. To accomplish this, it has been proposed that the meniscus at the entry nip of the pressure rollers be removed immediately after a pair of donor and receivers is fed through the rollers. In U.S. Pat. No. 3,271,187, an endless web surrounds the roller that contacts the back side of each receiver. The web has a diameter substantially greater than that of the roller it surrounds, is sufficiently rigid to retain its cylindrical shape, and includes a plurality of minute openings for entrapping the liquid comprising the meniscus as the rollers are rotated. The liquid entrapped in the openings is expelled from the openings by passing a gas through them. Such an arrangement may not be satisfactory. For example, it is relatively expensive, and the roller that contacts the back side of each receiver may become wet from any mist produced by gas-expelling the liquid from the openings in the web. In U.S. Pat. No. 3,357,337, the roller that contacts the back side of each receiver includes a plurality of radial openings connected with a vacuum source. When the rollers are rotated in contact with one another, following exit of a donor and receiver from between the rollers, the liquid comprising the meniscus is drawn from the roller nip through the radial openings, leaving the rollers substantially dry. Again, such an arrangement is expensive and may not be satisfactory in other respects. For example, the radial openings in the roller can produce a non-uniform surface pressure on a donor and receiver, which may undesirably affect imaging on the receiver.